This invention relates to a method and apparatus for controlling resistance welding which operates switchings at a high frequency and supplies an alternating current at a relatively low frequency to workpieces for resistance welding.
In recent years, a resistance welding machine using an inverter in the electric power supply has been widely employed. FIG. 15 shows an arrangement of a typical prior art inverter resistance welding machine. The inverter circuit 100 includes switching elements. In response to control pulses CP from the inverter control 102, the inverter circuit 100 switches the supplied direct voltage E into pulsed alternating current I.sub.1 (primary current) having a high frequency. The pulsed alternating current I.sub.1 from the inverter circuit 100 is supplied to the primary coil of the the welding transformer 104. As a result, the secondary coil of the transformer supplies a secondary pulsed alternating current which is proportional to the primary pulsed alternating current I.sub.1. The secondary pulses are converted into direct current I.sub.2 by means of the rectifier circuit 106 having a pair of diodes 106a and 106b. The direct secondary current or welding current I.sub.2 flows through workpieces W.sub.1 and W.sub.2 via welding electrodes 108 and 110.
Such an inverter resistance welding machine has also been used for simultaneous two point resistance welding (series welding) of workpieces, typically those of small metal for electronic components.
FIG. 16 illustrates a series welding. In FIG. 16, a pair of welding electrodes 108 and 110 make and keep pressure contact with workpieces W.sub.1 and W.sub.2 at spaced positions on one side of the workpieces by a welding force from a pressurizing system (not shown). During a weld time, a direct welding current I.sub.2 flows along a dotted line through a first welding electrode 108, a first workpiece W.sub.1, a first welding point Pa, a second workpiece W.sub.2, a second welding point Pb, the first workpiece W.sub.1 and a second welding electrode 110 in this order. The direct current flow causes metals at first and second welding points Pa and Pb of the workpiece W.sub.1 and W.sub.2 on the butt surface thereof to be melted by Joule heat. After the weld time, the welding points are solidified for metallurgically joining the workpieces.
The series welding using such a prior art inverter resistance welding machine has the problem that nuggets Na and Nb (deposit of fused metal) at welding points Pa and Pb are formed in uneven size due to Peltier effect. According to Peltier effect, a contact point between different metals, through which an electric current flows, not only undergoes a generation of Joule heat but also experiences a heat generation or absorption depending on the direction of the current.
In the series welding, the direction of the welding current I.sub.2 at the welding point Pa (contact point) between the workpieces (different metals) W.sub.1 and W.sub.2 is opposite to that at the second welding point Pb. Specifically, as shown in FIG. 16, the welding current I.sub.2 flows from the workpiece W.sub.1 to the workpiece W.sub.2 at the first welding point Pa whereas at the second welding point Pb the welding current I.sub.2 flows from the workpiece W.sub.2 to the workpiece W.sub.1.
Since the welding current I.sub.2 direction at the welding point Pa is opposite to that at the welding point Pb, one of the welding points, say, the first one Pa experiences a heat absorption of Peltier effect whereas the other, or the second welding point Pb experiences a heat generation of Peltier effect (though the same Joule heat may generate at these welding points Pa and Pb. As a result, the nugget Na at the first welding point Pa is formed in a relatively small size whereas the nugget Nb at the second welding point Pb is formed in a relatively large size, as shown in FIG. 16. Uneven size of the nugget Na and Nb means variations in joint strength at welding points and poor weld quality since the strength of the resistance welding is proportional to the nugget size.